Homeland Security Secretary John Kelly indicated on Tuesday that President Donald Trump's son-in-law and top adviser Jared Kushner and his communications with Russia are part of a wide-ranging probe by the Justice Department's special counsel.

Kelly defended Kushner at a Senate hearing amid reports that he attempted to establish a "back-channel" communication between Russia and Trump's presidential transition team. Under Democratic questioning, Kelly said "we have to make the assumption — and I will — that Jared Kushner is a great American."

Kelly said back-channel talks have been common in U.S. diplomacy, but indicated that the matter is under investigation by Special Counsel Robert Mueller as part of his probe into contacts between the Trump campaign and Russia.

White House senior adviser Jared Kushner, right, l
White House senior adviser Jared Kushner, right, looks on during a meeting between U.S. President Donald Trump, left, and leaders at the Gulf Cooperation Council Summit, at the King Abdulaziz Conference Center, in Riyadh, Saudi Arabia, May 21, 2017.

Montana Sen. Jon Tester, a Democrat, said he believes the conduct of the president's son-in-law is "unacceptable" and raises red flags. Noting that "there's all sorts of stuff going on here," Tester asked Kelly "to find out what the hell is going on."

As Tester persisted, Kelly repeated, "again, I think we have to make the assumption...."

Tester cut him off, asking: "But don't you think we should ensure that that assumption is correct?"

"Well sure," Kelly said. "And I think there are numerous investigations that are looking into this. I think it's part of the Bob Mueller investigation, and I think there's a number of congressional committees looking into it."

The exact scope of Mueller's investigation has so far been unclear, but he has broad authority.

Also Tuesday, a senior Democrat on the House Judiciary Committee wrote Deputy Attorney General Rod Rosenstein to clarify the scope and authority of Mueller's role and asked Rosenstein to ensure that the probe is free of political interference.

"It is of paramount importance that Mr. Mueller's investigation proceed unimpeded by any officials who may have a conflict of interest or other ethical bar, or by political interference of any kind," wrote Rep. Jerrold Nadler, D-N.Y.

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